The goal of the research proposed here is to use structure determination by X-ray crystallography and structure-guided functional experiments to determine the molecular basis of function for Drosophila neural (DN)-cadherin, an important intercellular adhesion protein. DN-cadherin plays an important role in patterning the Drosophila visual system. At later stages in the project, once appropriate DN-cadherin structures have been determined, we will employ the powerful methods of Drosophila genetic analysis to probe the mechanism of DN-cadherin function in vivo. The Drosophila genome encodes 3 classical cadherins, all of which are expressed in the eye. Only two of these cadherins function in adhesion, but it is only the adhesive activity of DN-cadherin is required for photoreceptor afferent-target selection. Although up to 5 distinct classical cadherins have been implicated in analogous functions in vertebrates, 12 distinct DN-cadherin isoforms are encoded by alternative splicing in the fly, suggesting the possibility that DN-cadherin may subsume multiple roles. Vertebrate classical cadherins are known to function in intercellular adhesion through binding at their extreme N-termini through a well-characterized binding mechanism. Vertebrate classical cadherin molecules presented on juxtaposed cells form an adhesive dimer by the reciprocal binding, or "swapping", of their Nterminal (3-strands. This strand-exchange is anchored by the insertion of highly conserved tryptophan residues from the exchanged strand from one partner cadherin into a conserved hydrophobic pocket in the partner molecule from the apposed cell. While key residues in this interaction are highly conserved among vertebrate species, alignments of vertebrate and invertebrate classical cadherins show that these residues are not conserved in the invertebrate molecules. This suggests that invertebrate cadherins, including DN-cadherin, are likely to function in adhesion by distinct mechanisms. Moreover, vertebrate and invertebrate classical cadherins are strikingly different in the overall number and architecture of their extracellular domains: mature vertebrate classical cadherins contain five extracelluar cadherin (EC) domains (EC1-5), whereas DN-cadherin contains between 10 and 15 (depending on splice form), and some EC domains appear to be significantly diverged from vertebrate EC domains. Like vertebrate cadherins, DN-cadherin may require proteolytic processing by a furin-like protease to achieve its mature form. PUBLIC HEALTH RELEVANCE: Knowledge of cadherin function and dysfunction is applicable to multiple facets of human health because of the role played by cadherins in the development of almost all solid tissues in vertebrate organisms. Cleft palate41, skin disorders42, and juvenile macular dystrophy eventually leading to blindness (HJMD)43 can be caused by mutations in human classical cadherin genes. Indeed, the study of DN-cadherin is particularly relevant in visual system development and disease.